The invention relates to a device for measuring the effective viscosity of a lubricant, provided with a motor, a sensor capable of being driven by the motor, to which sensor the lubricant to be measured can be supplied, and a measuring element measuring a motor parameter the value of which corresponds to the viscosity of the lubricant.
In a known device of this kind, the sensor consists of a cylindrical rotor capable of being driven by the motor and suspended in a vessel containing the lubricant to be measured. Between the rotor and the inside wall of the vessel, there is a comparatively large clearance. The viscosity is determined by measuring the power that must be supplied to the motor to obtain a given motor speed. With this known device, only an average viscosity of the lubricant can be measured, which is inadequate for the purpose of determining the quality of the lubricant for bearing applications, since in bearings a lubricant is generally present only in a thickness of less than one micron. Particles present in the lubricant, ranging in size from greater than 0 to 30-50 microns, and gases, will vary greatly influence the viscosity of the lubricant in such a lubricant film, while hardly at all affecting the average viscosity measured with the known device.
The object of the invention is to provide a device in which the difficulties mentioned are avoided by simple yet effective means.
For this purpose, the device according to the invention is characterized in that the sensor consists of a bearing having two parts movable relative to each other, one of which parts is capable of being driven by the motor, and in that a control means is arranged to control the motor speed in such a way that at the time of being driven by the motor, no roughness contacts occur between the parts of the bearing, while the measuring element determines the motor speed and indicates that speed as a viscosity observation.
In this way, a device is achieved whereby the viscosity of the lubricant can be measured under conditions prevailing in the bearing, so that a timely signal is obtained whenever the viscosity of the lubricant reaches an unacceptable level.
The operation of the device according to the invention is based on the realization that in the absence of roughness contacts between the parts of the bearing, the so-called film parameter .lambda. has a constant value (3), so that given the bearing constant C, which is a function of the viscosity and also depends on .lambda., by varying the motor speed so that no roughness contacts occur, the viscosity can be derived from said motor speed.
According to a simple embodiment of the invention, the control means is provided with a measuring means to measure the percentage contact time of the bearing, a comparison means comparing the percentage contact time with a predetermined reference value, and a motor control means that regulates the motor speed according to the output signal of the comparison means in such a way that the percentage contact time remains equal to the predetermined reference value.